I believe they're usually heavily subsidized. Possibly, if a private company could reduce costs enough they could operate a transit system without subsidies an

I believe they're usually heavily subsidized. Possibly, if a private company could reduce costs enough they could operate a transit system without subsidies and make money doing it, but I think ordinarily a private company running public transit is heavily reliant on government support. (Bus networks might be an exception, since they can use the roads that already exist and there isn't a huge up-front cost.) It sounds like the Boring company is mostly interested in providing the capability to make small tunnels cheaply. The car sled thing is just one possible way to use the tunnels, but if some city wants to do something different with them that's up to them.